This invention relates to methods and apparatus for measuring a patient""s transthoracic impedance, and more particularly, to techniques for measuring transthoracic impedance for use by a rate-responsive stimulation device.
Stimulation devices such as pacemakers are used to treat a variety of cardiac conditions. Some stimulation devices simply provide pacing pulses to a patient""s heart at a fixed rate. More sophisticated devices contain sensing circuitry that allows the stimulation devices to monitor a patient""s intrinsic signals. For example, some stimulation devices can monitor a patient""s atrial heartbeat signals and provide corresponding ventricular pacing pulses, which allows the patient""s cardiac output to be adjusted depending on the patient""s intrinsic atrial heart rate.
However, there are situations when the heart is not able to regulate its rate appropriately in response to physiological stress. This is known as chronotropic incompetence. Physiologically, the cardiac need of a patient varies depending on the patient""s physical activity level. Because of this, so-called rate-responsive stimulation devices have been developed that provide pacing pulses at a rate based on the patient""s level of exercise.
Some rate-responsive stimulation devices contain accelerometer-based activity sensors, which assess a patient""s level of physical activity by measuring the patient""s body movements. When the measured frequency and intensity of a patient""s movements are high, the patient""s heart is paced at a correspondingly high rate. Although this approach is generally satisfactory, many rate-responsive stimulation devices that use activity sensors are unable to clearly differentiate between body movements due to physical activity and body movements due to external sources (e.g., body movements experienced during an automobile ride).
Other rate-responsive stimulation devices use oxygen sensors to measure a patient""s blood-oxygen level. Rate-responsive stimulation devices that use oxygen sensors adjust the pacing rate to maintain a suitable oxygen level. However, oxygen sensors require the use of a special lead.
Another approach that has been used to assess a patient""s need for cardiac output is to attempt to determine the amount of air being inhaled and exhaled by the patient. Taking breaths deeply and frequently, for example when climbing the stairs, indicates that there is a high need for cardiac output. Accordingly, if a measure of a patient""s air usage can be provided, it can be used for rate-responsive pacing. One measure of a patient""s air usage is termed xe2x80x9cminute ventilationxe2x80x9d. Minute ventilation is the total volume of air moved in and out of the lungs in a minute. Transthoracic impedance is measured to calculate minute ventilation and is defined as a measure of the impedance across the chest cavity. More specifically, lungs that are filled with air have a higher impedance than lungs which are empty. Upon inhalation, impedance increases. Upon exhalation, impedance decreases. Minute ventilation is calculated based upon the formula:
Minute Ventilation=Tidal Volume*Respiration Rate
A rate-responsive device measures minute ventilation using the transthoracic impedance, computes a minute ventilation signal, and then compares the current minute ventilation with a long-term average of xe2x80x9cchange in minute ventilationxe2x80x9d to arrive at a required rate.
Consider, for example, FIG. 1 which is a graph of a parameter known as a rate response factor (RRF). The rate response factor can be used to set the expected change in pacing rate in response to increasing changes in minute ventilation during exercise. Each patient has a resting minute ventilation measurement and a maximum minute ventilation measurement. These are respectively indicated on the x-axis as xe2x80x9cRestingxe2x80x9d and xe2x80x9cPeakxe2x80x9d. Three lines are graphed in FIG. 1 and represent different RRFs relative to a particular patient to whom the graph corresponds. For example, the top line indicates a RRF that is too high for this patient because a maximum metabolic indicated rate is reached before the peak minute ventilation. The bottom line indicates a RRF that is too low for this patient because the maximum metabolic indicated rate is never reached. The middle line indicates an appropriate RRF for this patient because the maximum metabolic indicated rate is reached at the peak minute ventilation.
Before a minute ventilation sensor in a stimulation device can be activated, a baseline impedance measurement for a particular patient needs to be established. This is because the baseline impedance measurement is associated with a minimum pacing rate for that particular patient. The minimum pacing rate for the patient whose graph is shown in FIG. 1 is indicated as the minimum metabolic indicated rate. The minimum pacing rate when the patient is at rest might be around 60 ppm. The maximum pacing rate when, for example, the patient is exercising might be around 150 ppm. These two points are determined, respectively, by the baseline impedance signal (i.e. the signal when the patient is at rest) and the impedance signal when the patient is at maximum exercise. The line between these two points is determined by the rate responsive factor programmed into the stimulation device discussed above.
An advantage of monitoring the impedance of the chest cavity to assess cardiac need is that the stimulation device is less likely to be affected by body movements due to external sources and does not require the use of special leads.
One way for the stimulation device to measure body impedance is to apply a current signal of a known magnitude and waveform across the patient""s chest. The resulting voltage signal across the body can be measured by sensing circuitry. The impedance is calculated based on the known magnitude of the applied current signal and the measured magnitude of the voltage signal.
For example, many stimulation devices utilize a ventricular bipolar lead. A ventricular bipolar lead is a lead that is implantable in the ventricle. The lead is bipolar because it is fitted with two electrodes. One of the electrodes is used as the anode, and the other of the electrodes is used as the cathode. In these types of leads, a current signal for measuring impedance can be injected between a ring or coil electrode and the case or xe2x80x9ccanxe2x80x9d. A corresponding voltage signal can then be measured between a tip electrode and the case or can, as will be appreciated and understood by those of skill in the art.
One of the problems associated with minute ventilation sensors is that the lead or leads from which impedance measurements are ascertained can sometimes fail to operate as intended. This means that the impedance measurements that are utilized to provide rate-responsiveness can no longer be used to provide this functionality.
Accordingly, this invention arose out of concerns associated with providing improved stimulation devices and methods that provide improved minute ventilation sensors.
Methods and systems for measuring the transthoracic impedance of patient are described. Various embodiments provide stimulation devices and methods that can automatically adapt to different minute ventilation electrode configurations. This, in turn, permits minute ventilation functionality to continue, e.g. rate-responsive pacing, in spite of the fact that an electrode configuration has changed. Accordingly, minute ventilation functionality can automatically continue in an adaptive manner when a previously-available electrode configuration is no longer available for minute ventilation functionality.